Double Layer PCB Surface Treatment Process Characteristics, Use And Development Trends

- Oct 20, 2017-

With the human requirements for the living environment continues to improve, the current Double Layer PCB production process involved in the environmental problems is particularly prominent. The current topic of lead and bromine is the most popular; lead-free and halogen-free will affect the development of Double Layer PCB in many ways. Although the current point of view, Double Layer PCB surface treatment process changes are not great, it seems still relatively distant things, but it should be noted that: long-term slow changes will lead to a huge change. In the case of increasingly high environmental protection, Double Layer PCB surface treatment process will certainly change the future.

Purpose of surface treatment The most basic purpose of surface treatment is to ensure good solderability or electrical properties. Since the natural copper in the air tends to exist in the form of oxides, it is unlikely that long-term retention of copper, so the need for other treatment of copper. Although in the subsequent assembly, you can use strong flux to remove most of the copper oxide, but the strong flux itself is not easy to remove, so the industry generally do not use strong flux.

Now there are many Double Layer PCB surface treatment process, common is hot air leveling, organic coating, electroless nickel / dip gold, silver and immersion tin of these five processes, the following will be introduced one by one.

1. Hot air leveling; hot air leveling, also known as hot air solder leveling, it is in the Double Layer PCB surface coated with molten tin solder solder and heating compressed air by the whole (blowing) flat process, to form a layer of both copper oxidation, but also Can provide a good solderability of the coating. When the hot air is flattened, the solder and copper form a copper-tin intermetallic compound at the junction. The thickness of the solder to protect the copper surface is about 1-2 mil. Double Layer PCB hot air leveling to be immersed in the molten solder; air knife in the solder before the liquid blowing liquid liquid; air knife to solder on the copper surface of the moon to minimize and prevent solder bridge. Hot air leveling is divided into vertical and horizontal two, generally considered a better level, mainly horizontal hot air leveling coating is more uniform, can achieve automated production. Hot air leveling process of the general process: micro-corrosion → preheat → coating flux → spray tin → cleaning.

2. Organic coating; organic coating process is different from other surface treatment process, it is between copper and air as a barrier; organic coating process is simple, low cost, which makes it widely used in the industry. The early organic coated molecules are antirusted by imidazole and benzotriazole, the latest molecule is predominantly benzimidazole, which is chemically bonded to nitrogen-functional groups to copper on the Double Layer PCB. In the subsequent welding process, if the copper surface is only one layer of organic coating is not enough, there must be a lot of layers. This is why the chemical tank usually needs to add copper liquid. After coating the first layer, the coating adsorbs copper; the second layer of organic coated molecules are combined with copper until twenty or even hundreds of times of the organic coated molecules are assembled on the copper surface, which ensures that the secondary coating Flow welding. Experiments show that the latest organic coating process can maintain good performance during many lead-free soldering processes. The general process of organic coating process: degreasing → micro-corrosion → pickling → pure water cleaning → organic coating → cleaning, process control relative to other surface treatment process is easier.

3. Electroless nickel plating / immersion gold; Electroless nickel plating / immersion gold process is not as simple as organic coating, electroless nickel plating / immersion gold seems to wear thick armor to the Double Layer PCB; another chemical nickel plating / immersion gold process is not Like organic coating as a rust barrier layer, it can be used in the Double Layer PCB long-term use and to achieve good electrical properties. Therefore, the electroless nickel / leaching gold is wrapped in a thick layer of copper on the copper surface, good electrical strength of the gold alloy, which can protect the Double Layer PCB for a long time; In addition it also has other surface treatment process does not have the tolerance of the environment Sex. The reason for the nickel plating is that gold and copper will spread across each other, and the nickel layer can prevent the spread between gold and copper; if there is no nickel layer, gold will spread to copper in a few hours. Another benefit of electroless nickel plating / immersion gold is the strength of nickel, and only 5 microns of nickel can limit the expansion of the Z-direction at high temperatures. In addition, electroless nickel plating / immersion gold can also prevent the dissolution of copper, which will benefit lead-free assembly. The general process of electroless nickel plating / leaching process is: acid cleaning → micro-etching → prepreg → activation → electroless nickel plating → chemical leaching gold, there are six chemical tanks, involving nearly 100 kinds of chemicals, so the process control comparison difficult.

4. immersion silver; immersion silver process between organic coating and electroless nickel / immersion gold between the process is relatively simple and fast; unlike chemical nickel / immersion gold as complex, not to wear a thick layer of Double Layer PCB Of the armor, but it is still able to provide good electrical performance. Silver is a little brother of gold, even if exposed to hot, wet and polluted environment, silver is still able to maintain good solderability, but will lose luster. The immersion silver does not have the good physical strength of electroless nickel plating / immersion gold because there is no nickel below the silver layer. In addition, silver has a good storage, silver after a few years will not have a big assembly problem. The immersion silver is a replacement reaction, which is almost submicron coated with sterling silver. Sometimes the immersion process also contains some organic matter, mainly to prevent silver corrosion and eliminate the problem of silver migration; it is generally difficult to measure this thin layer of organic matter, the analysis shows that the weight of the organism is less than 1%.

5. Dip tin; since all current solder is based on tin, the tin layer can match any type of solder. From this point of view, dip tin technology has great prospects for development. However, tin before the tin after the tin process, in the welding process tin and tin migration will bring reliability problems, so the use of tin immersion process is limited. Later, in the immersion solution by adding organic additives, tin layer structure can make a granular structure, to overcome the previous problems, but also has good thermal stability and solderability. The dip tin process can form a flat copper-tin intermetallic compound, which makes the dip tin having the same good weldability as the hot air level without the hot air leveling headache flatness problem; the dip tin is also free of electroless nickel plating / Dip gold metal diffusion problem - copper tin intermetallic compounds can be firmly combined together. Dip tin plate can not be stored for too long, the assembly must be based on the order of immersion tin.

6. Other surface treatment process; other surface treatment process less application, the following look at the application of relatively more nickel plating and electroless plating process. Electroplating nickel is the originator of the Double Layer PCB surface treatment process, since Double Layer PCB appears it appears, after slowly evolving into other ways. It is in the Double Layer PCB surface conductor coated with a layer of nickel and then coated with a layer of gold, nickel is mainly to prevent the spread between gold and copper. There are two types of electroplating nickel: plating gold (gold, gold surface does not look bright) and hard gold plating (surface smooth and hard, wear, containing cobalt and other elements, gold surface looks more bright). Soft gold is mainly used for chip packaging when playing gold; hard gold is mainly used in non-welded electrical interconnection. Taking into account the cost, the industry often through the image transfer method for selective plating to reduce the use of gold. The current use of selective electroplating gold in the industry continues to increase, mainly due to the difficulty of electroless nickel plating / leaching process control. Under normal circumstances, welding will lead to electroplating gold brittle, which will shorten the service life, and thus to avoid welding on the gold plating; but the electroless nickel / leaching gold as the gold is very thin, and very consistent, brittle phenomenon rarely occurs The The process of electroless plating is similar to that of electroless nickel plating. The main process is through the reduction agent (such as sodium hypophosphite) to the palladium ions in the catalytic surface reduction to palladium, the new palladium can be a catalyst to promote the reaction, which can be any thickness of the palladium coating. The advantages of electroless palladium are good weld reliability, thermal stability, and surface smoothness.

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